A high-voltage discharge lamp, which comprises a discharge vessel encapsulating a pair of opposing electrodes and containing rare gas, a halide of light-emitting metal, and mercury, is used widely because it has relatively high efficiency and exhibits color rendering properties.
In recent years, a great demand has been made for small, high-efficiency light sources. High-pressure discharge lamps containing a halide of light-emitting metal are also now undergoing active development.
(Prior Art 1)
Jpn. Pat. Appln. KOKAI Publication No. 6-196131 discloses a structure comprising a ceramic discharge vessel, which contains a filler that can be ionized, including a metallic halide, and which surrounds a discharge space wherein the first and second electrodes are arranged. The discharge vessel has the first and second end sections connected to the ends of the center section that extends between the electrodes. Each end section surrounds a power-supplying conductor connected to the electrode, with some gap between it and the conductor. A seal made of ceramic-sealing compound is provided at a position where the power-supplying conductor protrudes outwardly from the end section. At least the first end section has an outer diameter smaller than the minimum outer diameter of the center section. The power-supplying conductor passing through the first end section has a part opposing the discharge space and being resistant to the halide and a part which facing away from the discharge space and exhibiting permeability to hydrogen and oxygen. In this high-voltage discharge lamp, the halide-resistant part of the power-supplying conductor extends into the first end section for a distance L1 that is 2 mm longer than the inner diameter of the first end section. And the power-supplying conductor passing through the second end section also has a part which opposes the discharge space and which is resistant to the halide.
According to the prior art 1, the part exhibiting permeability to the hydrogen and oxygen will not corroded even if is exposed to halogen or liberated halide. This is because the distance L1, for which the halide-resistant part connected to the part exhibiting permeability to hydrogen and oxygen extends into the first end section, is 2 mm longer than the inner diameter of the first end section.
In the prior art 1, the halide-resistant part of the power-supplying conductor is a molybdenum rod having a diameter of 0.7 mm or the like. The electrode is connected to a tip of the molybdenum rod. The electrode has been formed by winding a single tungsten wire having a diameter of 0.17 mm, around the free end portion of a tungsten rod having a diameter of 0.3 mm and a length of 3 mm, said free end portion being 0.8 mm long.
Some embodiments of the prior art 1 are disclosed, including one whose rated lamp power is an intermediate value of 70 W, another which is lighted at 50 W, and still another which is lighted at 150 W.
(Prior Art 2)
Jpn. Pat. Appln. KOKAI Publication No. 9-147803 discloses a structure of a high-voltage discharge lamp that comprises a light-emitting bulb and a pair of electrodes provided in the light-emitting bulb. The light-emitting bulb is made of translucent ceramics and contains light-emitting substance. The end sections of the light-emitting bulb have an outer diameter small than the maximum diameter of the light-emitting section of the bulb. At least one of the end sections of the light-emitting bulb is sealed with a seal member and a conductor. The conductor is an integral combination of the electrode and an external lead wire. The length L1 of the end section of the light-emitting bulb and the length L2 of the junction between the end section and the conductor, which are connected by seal member, are defined as: 2 mm.ltoreq.L2.ltoreq.20 mm, and 4 mm.ltoreq.L1-L2.ltoreq.20 mm.
The prior art 2 aims at preventing a reaction between the light-emitting substance and the seal member, thereby to solve the problems such as drop of the lamp voltage, lighting failure due to a leak and deterioration in lifetime.
Embodiments of the prior art 2 are disclosed. One embodiment has a lamp power of 150 W and comprises a light-emitting bulb having an internal volume of 0.9 cc, each section of which is 15 mm long. Another embodiment has a lamp power of 200 W and comprises a light-emitting bulb having an internal volume of 0.75 cc.
(Prior Art 3)
Jpn. Pat. Appln. KOKAI Publication No. 10-144261 discloses a structure of a ceramic discharge bulb for use in a high-voltage discharge lamp. The profile of the inner wall of the discharge bulb defines an inner chamber that contains a light-emitting filler. The inner chamber has one major axis and two ends having an opening each. Electrically conductive bushings are fitted in the openings of the ends in airtight fashion and electrically connected to the electrodes, respectively. The electrodes are arranged in the discharge bulb, opposing each other and spaced from each other by an inter-electrode distance (EA). The profile of the inner wall of the discharge bulb has a specific geometric form. Namely, the profile is composed of a cylindrical center section and almost semispherical end sections. The center section is straight and has a length (L) and an inner radius (R), and the end sections have a radius (R) equal to the radius of the center section. The length (L) of the cylindrical center section is smaller than or equal to the inner radius (R) thereof (namely, L.ltoreq.R). The inner length of the discharge bulb is at least 10% greater than the inter-electrode distance (EA) (that is, 2R+L&gt;1.1 EA). The diameter (2R) of the discharge bulb is at least 80% of the inter-electrode distance (EA) and at most 150% of the inter-electrode distance (EA) (that is, 1.5 EA.gtoreq.2R.gtoreq.0.8 EA).
The object of the prior art 3 is to render the temperature distribution uniform in the ceramic discharge bulb so that the bulb may be applied to all possible lamp postures.
According to the prior art 3, a special form is defined for the discharge bulb, whereby the wall load can be at most 45 W/cm.sup.2 for a rated power of 20 W and at most 25 W/cm.sup.2 for a high-power lamp.
The embodiments of the prior art 1 are all relatively large, high-voltage discharge lamps having a rated lamp power of 50 W or more. Thus, in the invention of the prior art 1, the electrodes are prepared independently of the power-supplying conductors, and the structure is adopted in which each electrode is connected to the tip of the halide-resistant part of the power-supplying conductor. This structure will encounter difficulty of assembling if it is applied to a small, high-voltage discharge lamp that has a lamp power of 35 W or less, for example 20 W.
In the prior art 1, a very narrow gap is provided between the inner surface of each end section of the ceramic discharge vessel and the halide-resistant part of the power-supplying conductor, and the shaft part of each electrode is located in the end section of the vessel. Therefore, each end section of the ceramic discharge vessel must be long enough to provide that narrow gap and to accommodate the shaft part of the electrode. That is, the end sections need to be longer than is necessary. The ceramic discharge vessel is inevitably long as a whole.
The prior art 1 was applied to such a small, high-voltage discharge lamp as described above. It was found extremely difficult to hold the coldest part at a low temperature to maintain the vapor pressure of the light-emitting metal at an optimal value, while keeping the temperature of the seal made of ceramic-sealing compound within a range to prevent the seal from being corroded by a halide.
Like the prior art 1, the prior art 2 is applied to relatively large, high-voltage discharge lamps having lamp powers of 150 W and 200 W. In these lamps, each electrode is connected to a conductor, forming an integral unit.
Only the relationship between the length L1 of the end section of the light-emitting bulb and the length L2 of the junction is defined to prevent reaction between the seal member and the light-emitting substance. In small, high-voltage discharge lamps, however, it is practically difficult for the prior art 2 to meet both the demand for the temperature of the seal made of ceramic-sealing compound and the demand for the temperature of the coldest part.
In the prior art 3, the discharge bulb is composed of a cylindrical center section and almost semispherical end sections connected to the ends of the center section. The length of the center section is defined on the basis of the radius R of the cylinder, and the inner length of the discharge bulb is defined on the basis of the inter-electrode distance, in order to render the temperature distribution uniform in the ceramic discharge bulb. In the embodiment shown in FIG. 1, each electrode is connected to a member 17 not shown. The lamp power of this embodiment is 70 W. This structure is similar to those of the prior art described above and can hardly be made smaller.
There is a demand, however, for a smaller high-voltage discharge lamp having a lamp power of 20 W or less, which is made of translucent ceramics and which has a long lifetime and a high efficiency.
To meet this demand, a small, high-voltage discharge lamp may be made, merely by reducing the sizes of the components of a conventional, relatively large, high-voltage discharge lamp, such as the discharge vessel and the electrodes. It was found, however, that a leak occurred at the seal in such a small lamp actually made, shortly after the lamp had been turned on. This is because the various modes of conveying heat to the seal from a heat source such as discharge plasma, i.e., heat conduction, convection and radiation, are unbalanced.
To realize small, high-voltage discharge lamps, the existing technology of high-voltage discharge lamps should therefore be reviewed thoroughly in order to create new specification that is suitable for small, high-voltage discharge lamps.